Composition and process for well cleaning

ABSTRACT

Disclosed is a microemulsion well treatment microemulsion that is formed by combining a solvent-surfactant blend with a carrier fluid. In preferred embodiments, the solvent-surfactant blend includes a surfactant and a solvent selected from the group consisting of terpenes and alkyl or aryl esters of short chain alcohols. The disclosed well treatment microemulsion can be used in well remediation, stimulation and hydrogen sulfide mitigation operations. Additionally, the well treatment microemulsion can be used in the production of benzothiophenes through interaction with hydrogen sulfide.

RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication No. 60/361,438 filed Mar. 1, 2002, entitled Composition andProcess for Well Cleaning, which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention generally relates to the production of petroleumproducts and more particularly to a composition and process forimproving the recovery of petroleum products from a subterraneangeological formation.

BACKGROUND OF THE INVENTION

For many years, petroleum products have been recovered from subterraneanreservoirs through the use of drilled wells and production equipment.During the production of desirable hydrocarbons, such as crude oil andnatural gas, a number of other naturally occurring substances may alsobe encountered within the subterranean environment.

For example, hydrogen sulfide (H₂S) is a highly toxic, colorless gasthat is produced during the decomposition of organic matter. In someareas, hydrogen sulfide gas is produced in large quantities during theretrieval of petroleum products. At relatively low concentrations (200ppm) and minimal exposure times, hydrogen sulfide gas can be lethal. Inareas prone to the production of hydrogen sulfide, drilling crews mustbe prepared to use detection and protective equipment at all times. Thecontamination of well sites from hydrogen sulfide gas is a significantenvironmental concern that requires extensive remediation. Additionally,during downstream processing, hydrogen sulfide is typically removed fromrefined products through expensive and waste-extensive procedures. Thecontrol and mitigation of hydrogen sulfide is a significant businessthat is strictly regulated throughout petroleum producing countries.

In addition to hydrogen sulfide, other undesirable downhole productsmust be managed during the production of hydrocarbons. For example,scale, paraffins, fines, sulfur, heavy oil tar by-products and waterblocks commonly accumulate in and around the formation, well casing,production tubing and recovery equipment. Alternatively, it may benecessary to remove injected fluids from the near wellbore area, such asdrilling fluids, cement filtrate, kill fluids, polymers and waterblocks. To maintain an efficient recovery of petroleum products, it isfrequently necessary to clean or remove these accumulations anddeposits.

The removal of unwanted deposits from the wellbore and productionequipment is generally referred to as “remediation”. In contrast, theterm “stimulation” generally refers to the treatment of geologicalformations to improve the recovery of hydrocarbons. Common stimulationtechniques include well fracturing and acidizing operations. Wellremediation and stimulation are important services that are offeredthrough a variety of techniques by a large number of companies.

Although a number of compounds and techniques are known in the priorart, there is a continued need for more effective methods and compoundsfor hydrogen sulfide mitigation, wellbore remediation, drillingoperations and formation stimulation.

SUMMARY OF THE INVENTION

The present invention includes a well treatment microemulsion that isformed by combining a solvent-surfactant blend with a carrier fluid. Inpreferred embodiments, the solvent-surfactant blend includes asurfactant and a solvent selected from the group consisting of terpenesand alkyl or aryl esters of short chain alcohols.

The inventive well treatment microemulsion can be used in wellremediation, stimulation, drilling operations and hydrogen sulfidemitigation procedures. Additionally, the well treatment microemulsioncan be used in the production of benzothiophenes through interactionwith hydrogen sulfide.

These and various other features and advantages that characterize thepresent invention will be apparent from a reading of the followingdetailed description and appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to the preparation and use of a welltreatment microemulsion in the management of undesirable downholeproducts encountered during the production of hydrocarbons fromsubterranean reservoirs. Unlike prior art cleaning and stimulationfluids, the well treatment microemulsions of the present invention arestablilized microemulsions that are formed by the combination ofsolvent-surfactant blends with an appropriate oil-based or water-basedcarrier fluid.

The solvent-surfactant blend generally includes a solvent, a surfactantand an alcohol. In a presently preferred embodiment, the solvent isselected from the group of unsaturated aliphatic cyclic hydrocarbonsknown as terpenes, including monoterpenes and diterpenes. In aparticularly preferred embodiment, the solvent is the monoterpened-limonene (C₁₀H₁₆). Terpenes, such as d-limonene, are preferred fortheir solvent qualities and biodegradability.

In an alternate embodiment, the terpene-based solvent is replaced withalkyl, cyclic or aryl acid esters of short chain alcohols, such as ethyllactate and hexyl ester. Ethyl lactate is a low cost, environmentallysafe solvent that can be manufactured from carbohydrates, such ascornstarch. Although acceptable for well remediation and stimulation,ethyl lactate is not generally recommended for use in hydrogen sulfidemitigation applications. It will also be understood that combinations ofdifferent solvents, such as d-limonene and ethyl lactate, are alsoencompassed within the scope of the present invention.

The selection of the surfactant component for the solvent-surfactantblend is determined by the type of carrier fluid selected. Water-basedcarrier fluids, such as fresh water and brine, are typically moreenvironmentally friendly and cost effective. Oil-based carrier fluids,such as diesel, kerosene and jet fuel may provide enhanced performancebut are generally more expensive and environmentally restricted.

If a water-based carrier fluid is chosen, the surfactant of thesolvent-surfactant blend should be capable of creating an oil-in-watermicroemulsion upon combination with an appropriate quantity of water.Preferred surfactants are biodegradable and have an HLB(hydrophile-lipophile balance) value ofbetween about 8-18. Presentlypreferred oil-in-water surfactants include one or more of the following:tween 40 (polyoxyethylene sorbitan monopalmitate), tween 60(polyoxyethylene sorbitan monostearate), tween 80 (polyoxyethylenesorbitan monooleate), linear alcohol alcoxylates, alkyl ether sulfates,dodecylbenzene sulfonic acid (DDBSA), linear nonyl-phenols, dioxane,ethylene oxide and ethoxylated castor oils such as PEG castor oil. Apreferred oil-in-water surfactant mixture includes polyoxyethylenesorbitan monopalmitate, ethoxylated castor oil and polyethylene glycol.

Alternately preferred oil-in-water surfactants can also includedipalmitoylphosphatidylcholine (DPPC), sodium 4-(1′heptylnonyl)benzenesulfonate (SHPS or SHBS), polyoxyethylene(8.6) nonylphenyl ether, aerosol O.T. (sodium bis-2-ethylhexylsulphosuccinate),A.O.T., tetraethyleneglycoldodecylether, sodium octlylbenzenesulfonate,O.B.S., SCS, IsalChem 145 (PO), sodium ether surfactant, E.O. sulonates(i.e., alkyl propoxy-ethoxysulfonate), alkyl propoxy-ethoxysulfate,alkylarylpropoxy-ethoxysulfonate and highly substituted benzenesulfonates (n-C12-oxylene-SO3-).

If an oil-based carrier fluid is chosen, the surfactant of thesolvent-surfactant blend should be capable of creating a water-in-oilmicroemulsion upon combination with oil. Preferred surfactants arebiodegradable and have an HLB value of between about 3-8. Presentlypreferred water-in-oil surfactants include span 40 (sorbitanmonopalmitate), span 60 (sorbitan monostearate) and span 80 (sorbitanmonooleate). A preferred water-in-oil surfactant mixture includessorbitan monopalmitate, ethoxylated castor oil and polyethylene glycol.

The alcohol component of the solvent-surfactant blend serves as acoupling agent between the solvent and the surfactant, therebystabilizing the microemulsion. The alcohol also lowers the freezingpoint of the well treatment microemulsion. Although isopropanol ispresently preferred, alternative suitable alcohols include midrangeprimary, secondary and tertiary alcohols with between 1 and 20 carbonatoms, such as t-butanol, n-butanol, n-pentanol, n-hexanol and2-ethyl-hexanol. Other freeze prevention additives can additionally oralternatively be added, such as detergent range alcohols ethoxylate,ethylene glycols (EG), polyethylene glycols (PEG), propylene glycols(PG) and triethylene glycols (TEG), with triethylene glycol beingpresently preferred.

The solvent-surfactant blend optionally includes a salt. The addition ofa salt to the solvent-surfactant blend reduces the amount of waterneeded as a carrier fluid and also lowers the freezing point of the welltreatment microemulsion. Among the salts that may be added for stabilityand co-solvent substitution, NaCl, KCl, CaCl₂, and MgCl are presentlypreferred. Others suitable salts can be formed from K, Na, Br, Cr, Csand Bi families.

After blending the solvents, surfactants and alcohols, it may bedesirable to form a diluted solvent-surfactant blend by adding a diluentbefore addition to the carrier fluid. Presently preferred diluentsinclude water and water and triethylene glycol (TEG) mixtures. Aparticularly preferred diluent is 90% by volume water and 10% by volumetriethylene glycol. It will be understood that upon addition of thediluent, the solvent surfactant blend may partially or completelyemulsify.

For oil-in-water well treatment microemulsions, the solvent-surfactantblend preferably includes about 36%-76% by volume of the preferredoil-in-water surfactant mixture (polyoxyethylene sorbitan monopalmitate,ethoxylated castor oil and polyethylene glycol), about 14%-54% by volumed-limonene and/or ethyl lactate and about 0%-10% isopropanol by volume.In a particularly preferred embodiment, the oil-in-watersolvent-surfactant blend includes about 56% by volume of the preferredoil-in-water surfactant mixture, about 34% by volume d-limonene, ethyllactate or combinations thereof, and about 10% by volume isopropanol.

In an alternativelypreferred embodiment, the oil-in-watersolvent-surfactant blend is diluted with about 50% by volume of diluent.The diluted solvent-surfactant blend preferably includes water and morepreferably includes about 45% by volume water and about 5% by volumetriethylene glycol. Accordingly, the diluted solvent-surfactant blendincludes about 27% byvolume of the preferred oil-in-water surfactantmixture, about 34% by volume d-limonene, about 5% by volume isopropanol,about 45% by volume water and about 5% by volume triethylene glycol.

For water-in-oil well treatment microemulsions, the solvent-surfactantblend preferably includes about 36%-76% by volume of the preferredwater-in-oil surfactant mixture (sorbitan monopalmitate, ethoxylatedcastor oil and polyethylene glycol), about 14%-54% by volume d-limoneneand/or ethyl lactate and about 0%-10% isopropanol by volume. In aparticularly preferred embodiment, the water-in-oil solvent-surfactantblend includes about 56% by volume of the preferred water-in-oilsurfactant mixture, about 34% by volume d-limonene, ethyl lactate or acombination of d-limonene and ethyl lactate, and about 10% by volumeisopropanol. The water-in-oil solvent-surfactant blend forms amicroemulsion upon combination with diesel or kerosene to form apreferred water-in-oil well treatment microemulsion.

In an alternatively preferred embodiment, the water-in-oilsolvent-surfactant blend is combined with about 0%-20% by volume of adiluent prior to adding the carrier fluid to form a diluted water-in-oilsolvent-surfactant blend. More preferably, about 5% by volume of diluentis added to the water-in-oil solvent-surfactant blend. The diluent caninclude water and more preferably includes about 45% by volume water andabout 5% by volume triethylene glycol. It will be understood that uponaddition of the diluent, the water-in-oil solvent-surfactant blend maypartially or completely emulsify.

The solvent-surfactant blends, dilute or concentrated, can be added tothe water and oil-based carrier fluids in sparing amounts to prepare thedesired well treatment microemulsions. For example, in manyapplications, as little as 0.2% -2% by volume of solvent-surfactantblend in water or oil based-carrier fluids will be sufficient. In otherapplications, however, it may be desirable to use a more concentratedwell treatment microemulsion. In such applications, the well treatmentmicroemulsion preferably includes about 0.5% to about 90% of theselected solvent-surfactant blend. Furthermore, it will be understoodthat in some applications, it may be desirable to apply thesolvent-surfactant blend, diluted or concentrated, without the additionof a carrier fluid. For example, the solvent-surfactant blend can bepumped downhole where it will incorporate water and water-basedmaterials to form the microemulsion in situ. Once formed, the welltreatment microemulsion can be pumped from the wellbore to the surface.

Although for the purposes of the present disclosure preferredembodiments of the well treatment microemulsions are described inconnection with well remediation, stimulation, acidizing operations,drilling operations and hydrogen sulfide mitigation applications, itwill be understood that the inventive well treatment microemulsions canbe used in additional, alternative applications. For example, it iscontemplated that the well treatment microemulsion could also be used toclean surface equipment and downhole equipment.

In well remediation applications, the selected well treatmentmicroemulsion is preferably injected directly into the wellbore throughthe production tubing or through the use of coiled tubing or similardelivery mechanisms. Once downhole, the well treatment microemulsionremedies drilling damage, fracturing fluid damage, water blocks andremoves fines, asphaltenes and paraffins from the formation andwellbore. The well treatment microemulsion also serves to thin heavyhydrocarbons, alleviate water blocks and lower pore pressure in theformation. If paraffin accumulation is significant, ethyl lactate orethyl lactate and d-limonene mixtures are preferred as solvents.

During drilling operations, the well treatment microemulsions can beadded to drilling fluids and injected into the wellbore through thedrill string. The well treatment microemulsion is effective at removingfines and debris from the wellbore created by the drilling process. Thesurfactant used in the solvent-surfactant blend should be selectedaccording to whether oil or water based drilling fluids are used.

The inventive well treatment microemulsions can also be used instimulation operations. In fracturing operations, proppant material canbe added to the microemulsion before injection downhole. Themicroemulsion is particularly effective at decreasing the density offilter cakes during high pressure injection of gelled fluids into thewellbore.

The well treatment microemulsions can also be used to deliver acidsduring acidizing operations. Acids commonly used include hydrochloric,acetic, formic, and hydrochloric-hydrofluoric acids. In a presentlypreferred embodiment, the selected solvent-surfactant blend (dilute orconcentrate) is combined with an acidified carrier fluid to prepare amicroemulsion suitable for acidizing operations. Preferably, themicroemulsion includes about 0.2%-5% by volume of the solvent-surfactantblend and about 3%-28% by volume of acid. In a particularly preferredembodiment, the microemulsion includes about 0.2%-5% of thesolvent-surfactant blend and about 15% by volume of hydrochloric acid.The concentration of the well treatment microemulsion in gelled fluidslowers the friction created by contact with conduits, therebyfacilitating the injection and withdrawal of the well treatmentmicroemulsion.

As mentioned above, the inventive microemulsions can also be used forhydrogen sulfide mitigation. In preferred embodiments, the welltreatment microemulsions are injected into the wellbore so that escapinghydrogen sulfide gas is “stripped” through the well treatmentmicroemulsions. Preferably, the inventive microemulsion is periodicallyinjected into problem wells to mitigate hydrogen sulfide production.Alternatively, the microemulsion can be injected downhole via capillarytubing on a continuous basis. In yet another alternate embodiment, thewell treatment microemulsion can be placed in a container that is placedin fluid communication with the hydrogen sulfide.

In a preferred embodiment, some or all of the water or oil-based carrierfluid is replaced with a known hydrogen sulfide scavenger. For example,many cyclic amines, such as triazines and hexamines, can be used as asolvent alone or in combination with water or oil-based carrier fluidsto further improve hydrogen sulfide mitigation.

The interaction between the well treatment microemulsions and thehydrogen sulfide neutralizes the hydrogen sulfide, leaving an inertsulfur compound as a product of the reaction. Significantly,benzothiophenes are also produced as a by-product of the reactionbetween the hydrogen sulfide and the well treatment microemulsions.Pharmaceutical researchers have recently discovered that benzothiophenescan be used as an intermediate in the synthesis of a number of usefulchemical compounds.

It is clear that the present invention is well adapted to carry out itsobjectives and attain the ends and advantages mentioned above as well asthose inherent therein. While presently preferred embodiments of theinvention have been described in varying detail for purposes ofdisclosure, it will be understood that numerous changes may be madewhich will readily suggest themselves to those skilled in the art andwhich are encompassed within the spirit of the invention disclosed andas defined in the written description and appended claims.

1. A well treatment microemulsion comprising: a solvent-surfactantblend, wherein the solvent-surfactant blend comprises: a solventselected from the group consisting of terpenes and alkyl or aryl estersof short chain alcohols; and a surfactant; a carrier fluid, wherein thecarrier fluid is water-based and the surfactant is an oil-in-watersurfactant mixture that includes an ethoxylated castor oil,polyoxyethylene sorbitan monopalmitate and polyethylene glycol; andwherein the solvent-surfactant blend is combined with the carrier fluidto produce the well treatment microemulsion.
 2. A well treatmentmicroemulsion comprising: a solvent-surfactant blend, wherein thesolvent-surfactant blend comprises: about 27% by volume surfactant,wherein the surfactant is an oil-in-water surfactant mixture thatincludes polyoxyethylene sorbitan monopalmitate, ethoxylated castor oiland polyethylene glycol; about 34% by volume d-limonene; about 5% byvolume isopropanol; about 45% by volume water; and about 5% by volumetriethylene glycol; a carrier fluid used to transport thesolvent-surfactant blend into a wellbore; and wherein the solventsurfactant blend is combined with the carrier fluid to produce the welltreatment microemulsion.
 3. A well treatment microemulsion comprising: asolvent-surfactant blend, wherein the solvent-surfactant blendcomprises: a solvent selected from the group consisting of terpenes andalkyl or aryl esters of short chain alcohols; and a surfactant, whereinthe surfactant is a water-in-oil surfactant mixture that includessorbitan monopalmitate, ethoxylated castor oil and polyethylene glycol;a carrier fluid, wherein the carrier fluid is oil-based; and wherein thesolvent-surfactant blend is combined with the carrier fluid to producethe well treatment microemulsion.
 4. A well treatment microemulsioncomprising: a solvent-surfactant blend, wherein the solvent-surfactantblend comprises: a solvent selected from the group consisting ofterpenes and alkyl or aryl esters of short chain alcohols; a surfactant;and wherein the solvent surfactant blend comprises: 25%-76% by volumesurfactant; 14%-54% by volume d-limonene; 0%-20% by volume isopropanol;and 0%-20% by volume water and triethylene glycol; a carrier fluid,wherein the carrier fluid is oil-based; and wherein thesolvent-surfactant blend is combined with the carrier fluid to producethe well treatment microemulsion.
 5. The well treatment microemulsion ofclaim 4, wherein the solvent-surfactant blend includes: about 56% byvolume surfactant, wherein the surfactant is a water-in-oil surfactantmixture that includes sorbitan monopalmitate, ethoxylated castor oil andpolyethylene glycol; about 34% by volume d-limonene; and about 10% byvolume isopropanol.
 6. A method for treating an oil or gas well having awellbore, comprising: forming a solvent-surfactant blend by combining asolvent and a surfactant, wherein the surfactant has ahydrophile-lipophile balance value of between 8-18 and wherein thesurfactant is an oil-in-water surfactant mixture that includes anethoxylated castor oil; preparing a microemulsion by combining thesolvent-surfactant blend with a water-based carrier fluid; and injectingthe microemulsion into the oil or gas well.
 7. The method of claim 6,wherein the oil-in-water surfactant mixture also includespolyoxyethylene sorbitan monopalmitate and polyethylene glycol.
 8. Amethod for treating an oil or gas well having a wellbore, comprising:forming a solvent-surfactant blend by combining a solvent and asurfactant, wherein the surfactant is a water-in-oil mixture ofethoxylated castor oil, sorbitan monopalmitate and polyethylene glycol;preparing a microemulsion by combining the solvent-surfactant blend withan oil-based carrier fluid; and injecting the microemulsion into the oilor gas well.
 9. A method for treating an oil or gas well having awellbore, comprising: forming a solvent-surfactant blend, wherein thesolvent-surfactant blend comprises: about 25%-76% by volume of anoil-in-water surfactant mixture that includes polyoxyethylene sorbitanmonopalmitate, ethoxylated castor oil and polyethylene glycol; about14%-54% by volume d-limonene; about 0%-20% by volume isopropanol; andabout 0%-50% by volume of water; preparing a microemulsion by combiningthe solvent-surfactant blend with a carrier fluid; combining themicroemulsion with proppant material; and injecting the microemulsionand proppant material into the wellbore to stimulate the oil or gaswell.
 10. A method for treating an oil or gas well having a wellbore,comprising: forming a solvent-surfactant blend; preparing amicroemulsion by combining the solvent-surfactant blend with a carrierfluid, wherein the microemulsion comprises: about 25%-76% by volume of awater-in-oil surfactant mixture that includes sorbitan monopalmitate,ethoxylated castor oil and polyethylene glycol; about 14%-54% by volumed-limonene; about 0%-20% by volume isopropanol; and about 0%-5% byvolume of water; combining the microemulsion with a drilling fluid,wherein the drilling fluid is oil-based; and injecting the microemulsionand drilling fluid into the oil or gas well during a drilling operation.11. A method for treating an oil or gas well having a wellbore,comprising: forming a solvent-surfactant blend; preparing amicroemulsion by combining the solvent-surfactant blend with a carrierfluid, wherein the microemulsion comprises: about 25%-76% by volume ofan oil-in-water surfactant mixture that includes polyoxyethylenesorbitan monopalmitate, ethoxylated castor oil and polyethylene glycol;about 14%-54% by volume d-limonene; about 0%-20% by volume isopropanol;and about 0%-50% by volume of water; combining the microemulsion with adrilling fluid, wherein the drilling fluid is water-based; and injectingthe microemulsion and drilling fluid into the oil or gas well during adrilling operation.
 12. A method for treating an oil or gas well havinga wellbore, comprising: forming a solvent-surfactant blend, wherein thesolvent-surfactant blend comprises: about 25%-76% by volume surfactant;about 14%-54% by volume d-limonene; about 0%-20% by volume isopropanol;and about 0%-50% by volume of water; combining the solvent-surfactantblend with an acidified carrier fluid to prepare a microemulsionsuitable for acidizing operations; and injecting the microemulsion intothe oil or gas well.
 13. The method of claim 12, wherein the acidifiedcarrier fluid includes one or more acids selected from the group ofhydrochloric, acetic, formic, and hydrochloric-hydrofluoric acids. 14.The method of claim 12, wherein the acidified carrier fluid includesabout 3%-28% acid by volume of the microemulsion.
 15. The method ofclaim 14, wherein the acidified carrier fluid includes about 15%hydrochloric acid by volume of the microemulsion.
 16. The method ofclaim 14, wherein the step of combining the solvent-surfactant blendwith an acidified carrier fluid further comprises: adding 0.2%-5%solvent-surfactant by volume of the microemulsion to the acidifiedcarrier fluid.